1. Field of the Invention
The present invention relates to monitoring turbulence in a continuous sheetmaking machine process, and more particularly, to a sensor for monitoring turbulence on the wire of a sheetmaking machine using wet end measurements.
2. State of the Art
In the manufacture of paper using a continuous sheetmaking machine, a web of paper is formed from an aqueous suspension of fibers (stock). Stock is dispersed from a dispensing unit referred to as a headbox onto a traveling mesh wire or fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dry section where steam heated dryers complete the drying process. The sheetmaking machine is essentially a de-watering, i.e., water removal system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction. Furthermore, in general, the elements of the system including the headbox, the web, and those sections just before the dryer are referred to as the "wet end". The "dry end" generally includes the sections downstream from the press. Papermaking elements and machines are well known in the art and are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking machines are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
Sheet formation (i.e., small-scale basis weight variation) is a basic sheet property that has a significant effect on optical and strength properties of the final sheet product. Sheet formation improves when average floc (i.e., grouped masses of particles) size or density decreases. Although, the stock approach system (i.e., elements prior to the headbox which provide the stock) and the headbox are key elements in delivering uniform and maximum dispersed stock, this is usually not adequate to produce a well-formed sheet. In particular what is further needed is defloccuation wherein clumping of the stock particles in a non-uniform manner is minimized. Defloccuation or dispersion can be generated in a number of ways such as by turbulence-inducing elements below the forming wire, by shear inducing elements above the fabric (e.g., dandy roll or top former), or by shaking the wire. Moreover, turbulence-induced in a sheetmaking machine affects particle orientation which also determines sheet strength.
In the art of making paper, sheet properties (such as sheet strength, thickness, and weight) are continually monitored and the sheetmaking machine controlled and adjusted to assure sheet quality and to minimize the amount of finished product that is rejected. This control is performed by measuring sheet properties at various stages in the manufacturing process which most often include basis weight, moisture content, and caliper (i.e., thickness) of the sheet, and using this information to adjust various elements within the sheetmaking machine to compensate for variations in the sheetmaking process.
Typically, a scanning sensor is used to perform basis weight measurements of the finished sheet at the dry end of the sheetmaking machine. Scanning sensors are known in the art and are described, for example, in U.S. Pat. Nos. 5,094,535, 4,879,471, 5,315,124, and 5,432,353. The basis weight measurements obtained from the scanner are used to control elements in the sheetmaking machine to adjust basis weight, and hence, paper quality.
To date, one property that has not been used to monitor on-line paper quality is wire turbulence. Instead, turbulence has been evaluated in an experimental environment to determine its effect on defloccuation and to determine optimum turbulence profiles. In particular, in the article "Turbulence Approach to Optimizing Fourdrinier Performance," by B. A. Thorp and R. A. Reese (Tappi Journal, March 1985, pp:70-73) turbulence is qualified by scale which is based on the number of peaks per unit area and by intensity which is based on the height of the peaks. Since these properties (i.e., scale and intensity) are based entirely on visual attributes, measurement of these properties are performed using equipment that will "stop" stock action to permit observation. Hence in this case, turbulence is evaluated using high intensity strobes and instant cameras with strobe flash units. Once photos are taken, they must be evaluated by a trained individual to count peaks per unit area and evaluate peak height. As can be imagined, turbulence measurements using this method are not immediately available and hence would not be suitable on-line information usable in a production environment. What would be desirable is to obtain on-line turbulence measurements so as to optimize papermaking system parameters in a production environment.